In the 1970s, the technique of percutaneous transluminal coronary angioplasty (PTCA) was developed for the treatment of atherosclerosis. Atherosclerosis is the build-up of fatty deposits or plaque on the inner walls of a patient's arteries; these lesions decrease the effective size of the artery lumen and limit blood flow through the artery, prospectively causing a myocardial infarction or heart attack if the lesions occur in coronary arteries that supply oxygenated blood to the heart muscles. In the angioplasty procedure, a guide wire is inserted into the femoral artery and is passed through the aorta into the diseased coronary artery. A catheter having a balloon attached to its distal end is advanced along the guided wire to a point where the sclerotic lesions limit blood flow through the coronary artery. The balloon is then inflated, compressing the lesions radially outward against the wall of the artery and substantially increasing the size of its internal lumen, to improve blood circulation through the artery.
Increasingly, stents are being used in place of or in addition to PTCA for treatment of atherosclerosis, with the intent of minimizing the need to repeatedly open an atherosclerotic artery. Although a number of different designs for stents exist in the prior art, all are generally configured as elongate cylindrical structures that can assume two different states, one having a substantially greater diameter than the other. A stent is implanted in a patient using an appropriate delivery system for the type of stent being implaced within the patient's arterial system. There are two basic types of stents--those that are expanded radially outward due to the force from an inflated angioplasty type balloon, such as the Palmaz-Schatz stent, the Gianturco-Roubin stent, and the Strecker stent, and those that are self expanding, such as the Maass double helix spiral stent, the Nitinol stent (made of nickel titanium memory alloy), the Gianturco stent, and the Wallstent. Problems with the Maass double helix spiral stent and the Nitinol stent have limited their use.
Stents are sometimes used following a PTCA procedure if the artery is totally occluded or if the lesions have occluded a previously placed surgical graft. Typically, a stent constrained within an introducer sheath is advanced to a site within the patient's artery through a guide catheter. For the balloon expanded type, after the introducer sheath is retracted, a balloon disposed inside the stent is inflated to a pressure ranging from about six to ten atmospheres. The force produced by the inflated balloon expands the stent radially outward beyond its elastic limit, stretching the vessel and compressing the lesion to the inner wall of the vessel. A self expanding stent expands due to spring force following its implacement in the artery, after a restraining sheath is retracted from the compressed stent, or in the case of the Nitinol version, the stent assumes its expanded memory state after being warmed above the transition temperature of the Nitinol alloy (e.g., above 30.degree. C.). Following the expansion process, if the balloon type is used, the balloon is removed from inside the stent and the catheter and other delivery apparatus is withdrawn. The lumen through the vessel should be substantially increased, improving blood flow.
After a stent or other endoluminal device is implanted, a clinical examination and either an angiography or ultrasonography morphological procedure is performed to evaluate the success of the procedure in opening the diseased artery or vessel. These tests are typically repeated periodically, e.g., at six-month intervals, since restenosis of the artery may become necessary. Due to the nature of the tests, the results of the procedure can only be determined qualitatively, but not quantitatively with any degree of accuracy or precision. It would clearly be preferable to monitor the flow of blood through the stent after its implacement in a vessel, both immediately following the stenosis and thereafter, either periodically or on a continuous basis. Measurements of volumetric rate and/or flow velocity of the blood through the stent would enable a medical practitioner to much more accurately assess the condition of the stent and of the artery in which the stent is implanted. Currently, no prior art mechanism is available that is implantable inside a blood vessel for monitoring the flow conditions through a stent.
Other parameters measurable within a stent or other type of endoluminal implant are also of interest and could be monitored using one or more appropriate sensors or transducers. For example, monitoring pressure at the distal and proximal end of the lumen in the implant and determining the differential pressure can provide an indication of fluid velocity through the lumen. Temperature can also be used to monitor fluid flow based by applying heat to the fluid within the lumen and monitor the rate at which the temperature of the fluid decreases as the fluid flows from the lumen of the implant. Integrated circuit (IC) transducers are currently known and available for sensing the levels of many different types of biochemical substances, such as glucose, potassium, sodium, chloride ions, and insulin. Any of these IC sensors could be provided in an endoluminal implant to monitors these parameters.
Since it is impractical to pass a conductor through the wall of an artery or vessel for long periods of time, use of a conventional sensor that produces signals indicative of flow through a stent, which must be conveyed through a conductor that extends through the wall of the vessel and outside the patient's body is not a practical solution to this problem. Also, any active flow indicative sensor must be energized with electrical power. Again, it is not practical to supply power to such a sensor through any conductor that perforates the vessel wall or that passes outside the patient's body. Battery power for such a sensor would quickly be exhausted and a battery would likely be too bulky to include inside an artery.
In addition to stents, the generic term endoluminal implant encompasses stent grafts, which are also sometime referred to as "spring grafts." A stent graft is a combination of a stent and a synthetic graft that is typically endoscopically implanted at a desired point in a vessel. Helically coiled wired comprising the stent are attached to the ends of the synthetic graft and are used to hold the graft in position. Sometimes, hooks are provided on the stent to ensure that the graft remains in the desired position within the vessel. Clearly, it would also be desirable to monitor the status of flow and other parameters through a stent graft, just as noted above in regard to a stent.
Endoluminal implants are used in other body passages in addition to blood vessels. For example, they are sometimes used to maintain an open lumen through the urethra, or through the cervix. A stent placed adjacent to an enlarged prostate gland can prevent the prostate from blocking the flow of urine through the urinary tract. Tracheal and esophageal implants are further examples of endoluminal implants. In these and other uses of an endoluminal implant, provision for monitoring parameters related to the status of flow and other conditions in the patient's body would be desirable. Information provided by monitoring such parameters can enable more effective medical treatment of a patient.